1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead having an extendable and retractable screw-in fixation mechanism.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices.
At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed. The implantation route and site are usually imaged in real time by fluoroscopy to confirm proper manipulation and placement of the lead.
A conventional cardiac stimulator lead normally consists of an elongated flexible tubular, electrically insulating sleeve that is connected proximally to a connector that is adapted to couple to the header of a cardiac stimulator, and distally to a tubular tip electrode. One or more ring-type electrodes may be secured to the sleeve at various positions along the length of the sleeve. The proximal end of the lead sleeve is connected to the connector by application of various biocompatible adhesives to various portions of the connector and the sleeve. The tip electrode ordinarily consists of a tubular structure that has an increased diameter portion that forms an annular shoulder against which the distal end of the lead sleeve is abutted. The exterior surface of the tubular structure is normally smooth as is the interior surface of the distal end of the lead sleeve.
In conventional active fixation tip electrodes, engagement with the endocardium is often achieved by projecting a corkscrew from the electrode. This is normally carried out by twisting the corkscrew. As the corkscrew spirals outward from the tip, the piercing point of the corkscrew pierces the endocardium, enabling the corkscrew to be screwed into the tissue by further twisting. The axial movement of the corkscrew relative to the tip electrode is usually accomplished by providing the electrode with a set of internal threads cut to match the pitch of the coils of the corkscrew. A stylet is inserted into the lead and temporarily coupled to the corkscrew. The stylet is twisted by hand to rotate the corkscrew.
Conventional open lumen leads of both the active and passive fixation varieties are subject to the influx of body fluids. Some fluid influx is usually expected, particularly in the period immediately following implantation when inflamation is most pronounced and fibrous in-growth is not established enough to provide a natural barrier to fluid flow. However, some leads are subjected to heavy influx as a result of blood disorders such as hemophilia, unexpected and prolonged inflammation, or other causes. Heavy and/or prolonged influx may harm the lead. To counter the potentially deleterious effects of fluid influx, conventional open lumen leads frequently include a washer or gasket within the tip electrode to restrict the influx of body fluids into the leads. These gaskets are molded with a central opening of fixed diameter to accommodate the corkscrew.
There are several disadvantages associated with conventional active fixation leads. It is often difficult for the implanting physician to verify both the proper deployment of a corkscrew from the tip electrode, and the successful engagement of myocardial tissue by the corkscrew. The difficulty stems from the fact that past and current corkscrews are too small to be readily perceived via fluoroscopy. In circumstances where a conventional corkscrew fails to deploy in situ and there is no visual verification of the problem, a physician may needlessly persist in twisting a stylet in an attempt to extend the corkscrew.
In addition to presenting difficulties in detecting corkscrew deployment, conventional leads do not provide visual verification of endocardial penetration by the corkscrew. The problem also stems from limitations in X-ray imaging. As a consequence, the most common method of verifying a proper engagement of the endocardium by the corkscrew is by touch. Following deployment of the corkscrew, the physician applies a gentle, axial, tensile force on the lead connector. An absence of appreciable longitudinal movement of the lead is an indication that the corkscrew has successfully penetrated and engaged the tissue. However, a sudden longitudinal movement of the lead is an indication that the corkscrew either did not engage enough tissue or did not engage any tissue at all. In such circumstances, the physician must retract the corkscrew, reposition the tip of the lead proximate the targeted tissue, and redeploy the corkscrew. This process may be very time consuming, particularly where very precise electrode positioning is medically indicated and the targeted tissue is difficult to reach, e.g. requires complex bending and manipulation of the stylet.
The problem of tissue engagement verification may be aggravated by other aspects of conventional tip electrode and corkscrew design. In most conventional leads, the corkscrew is deployed by a set of internal threads in the tip electrode. The threads extend from some point within the electrode to the opening at the distal end of the electrode from which the corkscrew deploys. A by-product of this design is that the corkscrew deploys as soon as the stylet is twisted. This may not be problematic where the tip is positioned and maintained in close contact with the targeted tissue. However, if the tip is not bearing directly against the targeted tissue or not positioned within a fraction of the total length of the corkscrew at the time the stylet is twisted, the corkscrew may deploy and either not engage any tissue at all or only penetrate a small distance into the tissue. In the former situation, the corkscrew will have to be retracted and second attempt made. In the latter scenario, two undesirable outcomes may result. First, a less than optimum amount of tissue penetration may result. Second, minimal tissue penetration by a fully extended corkscrew may result in the conducting tip of the electrode having only intermittent physical contact with the targeted tissue or no contact at all.
As noted above, many conventional leads incorporate a washer. A drawback associated with conventional washer design is fixed aperture size. Where, as is often the case, the washer is coaxially located with the corkscrew, the aperture must be made large enough to accommodate the outer diameter of the corkscrew. While such washers provide some restriction to fluid influx, their capability in this regard is limited by their permanently sized apertures.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.